Sudoku: 6x6 Medium

6×6 sudoku, medium: the size where scanning stops being enough

Thirty-six cells and the digits 1 to 6, in boxes two rows deep and three columns wide rather than square. The rules are the familiar ones. Each digit once per row, once per column, once per box.

Medium gives you between 18 and 22 clues. On an easy 6×6 you would get 22 to 26, and that difference of a handful of digits is the whole point of the setting — below about 22 clues, pure scanning runs out and you have to start reasoning about where a digit cannot go.

The 2×3 box is the thing that catches people

On a 9×9 the boxes are square, so a box spans three rows and three columns and the symmetry lets you think about rows and columns interchangeably.

A 2×3 box does not. It spans two rows and three columns, which means a box overlaps three cells of a row but only two of a column, and the scanning technique that works down a column is weaker than the one that works along a row. Children arriving from 4×4 puzzles notice this as the puzzle "feeling lopsided", and it is the single most useful thing to point out when they get stuck.

Uniqueness, and why the clue count varies

Every puzzle starts as a complete solved grid. Clues are then removed one at a time, and after each removal the grid is re-solved: if a second solution has appeared, the clue goes back. Removal stops when the target clue count is reached or nothing more can go.

That is why two medium sheets are not identical in difficulty. One may sit at 18 clues and another at 22, both correctly labelled medium, because the number that could be removed depends on the grid.

Six to a page

The default sheet carries six puzzles. That suits a class, a car journey, or a week of one a day. The solutions toggle adds a page with all six completed.

Where 6×6 sits in the progression

It is the rung between the 4×4 and the full 9×9, and it is the rung people skip.

A child who can do 4×4 puzzles fluently is doing something closer to pattern-matching than deduction, because four digits in sixteen cells rarely require more. Nine by nine is a long way from there. The 6×6 introduces genuine elimination, non-square boxes, and a grid big enough that you have to keep track of what you have already worked out, all while still finishing inside ten minutes.

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